It is well known to provide heat exchangers in which an impervious metal wall separates the fluids and conducts heat therebetween. It is also known to provide outward projections on one or both sides of such a metal wall to increase the area contacted by the fluid. For example, U.S. Pat. No. 166,461 discloses spaced concentric metal pipes in which metal projections are provided on the inner and outer sides of the inner pipe. In U.S. Pat. No. 2,112,743 metal screening is arranged on one or both sides of a metal wall separating fluids.
A construction of heat exchanger is also known from U.S. Pat. No. 2,825,210 which discloses a plurality of metal sheets spaced apart by interposing blocks of rubber between adjacent sheets to divide their areas into side-by-side flow paths for different fluids, these flow paths being parallel to the flat surfaces of the metal sheets.